1. Field of the Invention
The present invention relates to an optical scanning apparatus, more particularly, although not exclusively, the present invention relates to an image forming device using an optical scanning apparatus.
2. Description of the Related Art
Conventionally, in an optical scanning apparatus, such as a laser beam printer (LBP), a light flux optically modulated and emitted by a light source in accordance with an image signal is periodically deflected by an optical deflector constituted, for example, by a polygon mirror, and converges as a spot on a photosensitive recording medium (a photosensitive drum) by an imaging optical system having a fθ characteristic. Then, the photosensitive recording medium is optically scanned to perform image recording.
FIG. 16 is a schematic diagram showing the essential portion of a conventional optical scanning apparatus.
In FIG. 16, a scattering light flux, which can be emitted by a light source 1, is converted, by a collimating lens 3, into substantially parallel light fluxes that are limited by a diaphragm 2 and enter a cylindrical lens 4 having a predetermined refracting power in the sub-scanning direction. Of the substantially parallel light fluxes that enter the cylindrical lens 4, the light fluxes that are discharged are unchanged in cross section in the main scanning direction, while in cross section in the sub-scanning direction, the light fluxes are converged and condensed substantially to a line image on a deflection face (a reflection face) 5a of a deflection portion 5, which is formed of a polygon mirror.
Then, the light fluxes deflected by the deflection face 5a of the deflection portion 5 are guided, via an imaging optical system 6 having an fθ characteristic, to a photosensitive drum face 8, a target scanning face. Thereafter, the deflection portion 5 is rotated in a direction indicated by an arrow A to optically scan the photosensitive drum 8 in a direction indicated by an arrow B. In this manner, image data are recorded.
For such an optical scanning apparatus, a method is discussed in Japanese Patent Laid-Open Publication No. Hei 10-68903 (corresponding to U.S. Pat. No. 6,094,286). According to this method, a magnification change and a focus change due to a temperature fluctuation in an optical scanning apparatus are corrected by I.) a power change between a plastic lens and a diffraction section, where the plastic lens is arranged between a polygon mirror and the face of a photosensitive drum, and II.) a wavelength change for a semiconductor laser.
Furthermore, conventionally, an infrared semiconductor laser (780 nm) or a red semiconductor laser (675 nm) has been employed as a light source. However, in response to a demand for higher resolutions and in order to obtain a tiny spot, an optical scanning apparatus has been developed that employs a short wave laser having an oscillating wavelength of 450 nm or shorter. A short wave laser, compared with a conventional optical scanning apparatus employing an infrared laser, can obtain a small spot having half the conventional diameter, while for an imaging optical system, the conventional emission F number is maintained. An optical scanning apparatus that employs a short wave laser is discussed, for example, in Japanese Patent Laid-Open Publication No. Hei 11-281911.
The use of a short wave laser having a wavelength of 450 nm or shorter is not discussed in Japanese Patent Laid-Open Publication No. Hei 10-68903 (U.S. Pat. No. 6,094,286).
An optical scanning apparatus that employs a short wave light source of 500 nm or shorter is discussed in Japanese Patent Laid-Open Publication No. 2002-303810. According to this optical scanning apparatus, an imaging position shift (a magnification color difference) that occurs in the main scanning direction, due to a wavelength change, is reduced by optimizing the power arrangement in the main scanning direction for at least one plastic lens and at least one diffraction surface, which are located between a polygon mirror and a photosensitive drum and which constitute an imaging optical system.
In order to use an optical scanning apparatus to perform accurate image data recording, the field curvature should be appropriately corrected across the entire scanning face, the isokinetic distortion characteristic (fθ characteristic) should be present between an angle of view θ and an image height Y, and the spot diameter on an image face should be uniform for each image height.
However, with an optical scanning apparatus that uses a short wave light source having a wavelength of 450 nm or shorter to obtain a spot having a diameter half that of a conventional spot, the following problem is encountered.
FIG. 17 is a cross-sectional view of an optical scanning apparatus, taken in the main scanning direction, that employs as a light source (not shown) a gallium nitride bluish violet laser (wavelength λ=405 nm). Two lenses in FIG. 17 are nonspherical lenses (e.g., made of plastic or other optical material as known by one of ordinary skill in the relevant art).
For an optical scanning apparatus using a conventional infrared laser, the spot diameter in the main scanning direction is set as 60 μm and the spot diameter in the sub-scanning direction is set as 70 μm. The depth of focus is shown in graphs in FIGS. 18 and 19 while slice levels are set as 75 μm in the main scanning direction and 85 μm in the sub-scanning direction. About ±5.0 mm is the distance in the defocusing direction on the image face, across the entire image height in the main scanning direction, on which a spot having a diameter of 75 μm or smaller is obtained. Similarly, about ±7.0 mm is the distance in the defocusing direction on the image face, across the entire image height in the sub-scanning direction, on which a spot having a diameter of 85 μm or smaller is obtained.
However, for an optical scanning apparatus using a short wave laser of 405 nm, the spot diameter set in the main scanning direction is 30 μm and the spot diameter set in the sub-scanning direction is 37.5 μm, when the slice levels set in the main scanning direction are 37.5 μm and in the sub-scanning direction are 42.5 μm, the depths of focus are about ±1.3 mm and about ±2.2 mm respectively in the main scanning direction and in the sub-scanning direction, as shown in FIGS. 20 and 21. This is because the depths of focus are proportional to the wavelength.
On the other hand, an inexpensive plastic lens that can quite arbitrarily be shaped is frequently employed as a scanning lens for an imaging optical system. The change rate for the refracting power of a plastic lens is higher than that of a glass lens, but as heat is generated by a polygon motor or a circuit board, for example, the refracting power is reduced, and accordingly, the focal position on a target scanning face is shifted. According to the conventional optical scanning apparatus shown in FIG. 17, when the environment temperature was changed 25° C., for example, the focus of the scanning lens (a plastic lens) was shifted away from the center of an image at a focal distance of 0.9 mm in the main scanning direction (dm) and at a focal distance of 1.3 mm in the sub-scanning direction (ds). In the graph in FIG. 22, the focal shifting of the optical lens (a plastic lens) that occurred when the temperature was raised 25° C. is plotted for each image height.
Efforts at spot size reduction can be made more difficult by focal shifting that can occur due to manufacturing errors during the production of an optical scanning apparatus that employs a short wave laser (450 nm or shorter), or by focal shifting that exceeds the depth of focus occurring in an environment wherein the temperature is raised Therefore, for an optical scanning apparatus that uses a short wave laser, the precision with which parts are produced and assembled can exceed that which is conventionally required, and the apparatus can exhibit superior environmental stability.
In Japanese Patent Laid-Open Publication No. Hei 11-281911, no reference is made to the above described shortcomings and the optical scanning apparatus disclosed in this document is available in a special environment, constantly maintained at a steady temperature, and this has a low degree of practicability.
According to the optical scanning apparatus disclosed in Japanese Patent Laid-Open Publication No. Hei 10-68903 (U.S. Pat. No. 6,094,286), a diffraction section is arranged between a polygon mirror and a photosensitive drum to compensate for focal shift that occurs due to an environmental temperature change.
Since a greater chromatic aberration occurs for the diffraction section, compared with the scanning lens, an optical design is required that takes into account not compensation for the temperature but also for the chromatic aberration.
In Japanese Patent Laid-Open Publication No. Hei 10-68903 (U.S. Pat. No. 6,094,286), a diffraction surface is also provided along an optical path between a light source and the polygon mirror; however, the use of a short wave laser (450 nm or shorter) for the optical scanning apparatus is not disclosed in Hei 10-68903.
That is, when a diffraction section is arranged between the polygon mirror and the photosensitive drum to compensate for a focal change that occurs due to the effect environmental temperature changes have on the scanning lens (a plastic lens), which is positioned between the polygon mirror and the photosensitive drum, a problem that a chromatic aberration is increased occurs.
According to the optical scanning apparatus in Japanese Patent Laid-Open Publication No. 2002-303810 that employs a short wave laser (450 nm or shorter), a diffraction surface is provided between a polygon mirror and a photosensitive drum to compensate for a magnification chromatic aberration that occurs in a scanning lens (a plastic lens). With this arrangement, focal changes due to environmental temperature changes can not be compensated for.
Further, in Japanese Patent Laid-Open Publication No. 2002-303810, the provision of a diffraction surface between a light source and the polygon mirror is not discussed.
Furthermore, for a collimating lens that converts laser light fluxes into substantially parallel light fluxes, the dispersive characteristic of the lens material is degraded as the wavelength is shortened, and the chromatic aberration becomes a possible problem.
In addition, for a multi-beam optical scanning apparatus for which the number of light sources (or light-emission points) is increased in accordance with a request for an increase in processing speed, a problem that can occur is when the wavelengths of the light sources do not match, one of the light sources can be out of focus, even though the others are in focus. Thus, even when light sources are selected so that the same wavelength difference is obtained, as in the case of a conventional infrared laser, an aberration can occur because, in the short wave region, the dispersive characteristic of the lens material is not satisfactory to avoid aberration effects.
As multi-beam optical scanning apparatuses is discussed in Japanese Patent Publication No. Hei 6-82172 that employs a polarized beam splitter and an apparatus that employs a monolithic multi-beam light source where multiple light emission points are present.
The above described conventional examples do not discuss any solutions for problems that have occurred when a short wave light source has been employed.